Nuclear magnetic resonance (NMR) refers generally to one form of gyromagnetic spectroscopy which is conducted to study nuclei that have magnetic moments. Any nucleus which possesses a magnetic moment attempts to align itself with the direction of the magnetic field in which it is located. The nucleus precesses around the direction of the magnetic field at a characteristic angular frequency known as the Larmor frequency. The Larmor frequency is dependent upon the strength of the magnetic field and on the properties of the specific nuclear species.
Subjecting human tissue to a uniform magnetic field will cause the individual magnetic moments of the paramagnetic nuclei in the tissue to attempt to align with this magnetic field, but will precess about it in random order at their characteristic Larmor frequency. If the tissue is irradiated with a magnetic field (excitation field B.sub.1) which is in the perpendicular plane relative to the direction of the polarizing field B.sub.z, and which is near the Larmor frequency, the net aligned moment M.sub.z, can be rotated into the perpendicular plane (x-y plane) to produce a net transverse magnetic moment M.sub.1 which is rotated in the x-y plane at the Larmor frequency. Once the magnetic field (excitation field B.sub.1) is terminated, an oscillating sine wave (referred to as an NMR signal) is induced in a receiving coil by the rotating field produced by the transverse magnetic moment M.sub.1. The frequency of the signal is the Larmor frequency, and its magnitude is determined by the magnitude of M.sub.1.
A weak nuclear magnetic resonance generated by the precessing nuclei may be sensed by an RF coil and recorded as an NMR signal. From this NMR signal, a slice image may be derived according to well known reconstruction techniques. The quality of the image produced by the MRI techniques is dependent, in part, on the strength of the NMR signal received from the precessing nuclei. For this reason, an independent RF receiving coil is placed in close proximity to the region of interest of the imaged object to improve the strength of this received signal. Such coils are referred to as "local coils" or "surface coils".
"Whole body" NMR scanners are sufficiently large to receive an entire human body, and to produce an image of any portion thereof. Such whole body scanners may employ an excitation coil for producing the excitation field and a separate receiver coil for receiving the NMR signal. The excitation coil produces a highly uniform excitation field throughout the entire area of interest, whereas the receiver coil is placed near the immediate area of interest to receive the NMR signal.
The smaller area of the local coils permit them to accurately focus on NMR signal from the region of interest. The smaller size of the local coil makes it important that the local coil be accurately positioned near the region of interest. For "whole volume" coils, employing two antenna loops to receive the NMR signal from a volume defined between the loops, accurate positioning of the coils is particularly important. This leads to the development of local coils which conform to the anatomy of interest, yet function to permit ease of use.